1. Field of the Invention
The present invention relates to a method of manufacturing an image bearing member, the thus obtained image bearing member, and an image forming apparatus, and a process cartridge using the image bearing member.
2. Discussion of the Background
Organic photoconductors, which mainly contain organic material, are widely used as image bearing members in electrophotography field in terms of cost, productivity, free latitude of material selection, impact on global environment, etc. Such organic photoconductors are formed of a photosensitive layer mainly containing photosensitive material and typified into a single layer type in which a single layer assumes both a charge generation function and a charge transport function, and a laminate type having a charge generation layer assuming a charge generation function and charge transport layer assuming a charge transport function.
The mechanism of forming latent electrostatic images on an image bearing member of a function separated laminate type is that when a uniformly charged image bearing member is irradiated with light, the light passes through the charge transport layer and is absorbed in charge generation material in the charge generation layer to generate charges (a pair of charges). One in the pair is infused into the charge transport layer at the interface between the charge generation layer and the charge transport layer, transferred into the charge transport layer by an electric field to the surface of the image bearing member and then neutralize the surface charge provided by charging to form a latent electrostatic image. The organic photoconductors having such a laminate structure is advantageous in terms of the stability of the electrostatic characteristics and durability and thus currently the mainstream of the image bearing members.
With an advance in improvement on the development material or the image forming apparatus itself in addition to the image bearing member, an image forming apparatus using an organic photoconductor has been rapidly improved toward full colorization and high speed performance. The use application of printing has been accordingly diversified and the image forming apparatus in the electrophotography has been used in quick printing in recent years. In the quick printing filed, it is required to maintain the image quality free from image deficiency during repetitive printing of the same image or document. To meet this requirement, art image bearing member needs to simultaneously have excellent mechanical durability, i.e., a strong surface for abrasion and damage, and excellent electrostatic durability, i.e., being free from charge reduction, rise in residual voltage and/or sensitivity deterioration, for repetitive printing over an extended period of time.
For example, unexamined published Japanese patent application No. (hereinafter referred to as JOP) S56-48367 describes (1) a technology in which a curable binder is used in the surface layer; JOP S64-1728 describes (2) a technology in which a curable binder is used in the surface layer; and JOP H04-281461 describes (3) a technology in which a filler is dispersed in the surface layer. Among these technologies, the technology of using a curable binder improves abrasion resistance and damage resistance due to the curing but is not sufficient in the conditions of the quick printing field which is severer than those of the typical printing in office environments. In the technology of curing a surface layer, material contained in the surface layer not involved in the reaction causes curing inhibition, and thus monomers may remain non-reacted in the surface layer, which may lead to a rise of the residual voltage, charge reduction, degradation of anti-chemical property. In addition, promotion of curing reaction to improve the hardness of a surface layer leads to a significant rise of the residual voltage and prevents a good combination of the mechanical strength and electrostatic durability. The technology of using a charge transport polymer improves anti-abrasion property in some degree but is not good enough for use in the quick printing field. In addition, polymerization and refinement of such a charge transport polymer is difficult and thus the charge transport polymer having a high purity is not easy to obtain. Therefore, this technology is not good enough in terms of the electrostatic durability. Furthermore, there are manufacturing problems such that the liquid application has a high viscosity. The technology with regard to filler dispersion improves the abrasion resistance but again is not sufficient in the quick printing field. The mechanical strength is improved by containing a filler but detached filler may damage the surface of the image bearing member, which may lead to occurrence of filming or attachment of foreign objects as the damage develops. In addition, the charge trap site existing on the surface of the filler raises the residual voltage, which easily reduces the image density. As described above, these typical technologies improve the abrasion resistance of an image bearing member but cause electrostatic deterioration, which results in the occurrence of image deficiency, etc. Thus, these technologies are not sufficient in application use in the quick printing field.
Also, Japanese patent application No. (hereinafter referred to as JP) 3734735 describes a technology in which a surface layer is formed by spray coating a liquid application prepared by using a resin, filler and a solvent which dissolves a resin for use in the surface of a photosensitive layer. JOP H07-5703 describes a technology to improve chargeability, dark decay, and the electrophotographic property during repetitive use.
Furthermore, there is described a method in which these technologies are combined by a curable resin in the surface layer and containing a filler therein. For example, JP 2821318 describes a method in which a protection layer is formed by using a liquid application in which electroconductive metal oxide particulates are dispersed in a specific curable acryl based monomer. Also, for example, JOP 2000-330313 describes a technology in which particulates and a binder resin in which a charge transport material containing a hydroxyl group or carboxyl group is cross linked with a block polymerized isocyanate are contained in a surface layer. Furthermore, there is described a method (e.g., in JOP 2005-99688) in which filler particulates are dispersed in a cross-linked resin layer formed by curing a radical polymerizable monomer having three or more functional groups without a charge transport structure, and a radical polymerizable compound having a charge transport structure. In addition, there is described a method (e.g., in JOP 2006-330086) in which a thermosetting binder resin, a charge transport material having a cross-linkable functional group, and electroconductive particulates are contained. Containing a filler in a cured resin is an efficient way to prevent detachment of the filler and improve the anti-abrasion property. When electroconductive particulates are used as the filler, such particulates are added to control the resistance of the surface layer, but have a great effect on improvement on the abrasion resistance. However, when a material not involved in the curing reaction of the cured resin is mixed, such a material inhibits curing reaction, in which case such effects are not obtained.
Therefore, a method in which a filler is contained in a cured resin is anticipated to have a great effect but is extremely difficult to realize.
For example, there is a problem with the dispersion property of a filler. To improve the dispersion property of a filler, JP 3802787 describes a method of using a moisture dispersion agent of the unsaturated poly carboxylic acid type having an acid value of from 30 to 400 mgKOH/g. This is extremely an effective way in the present invention. However, In JP 3802787, a thermoplastic resin is used as the binder resin while a cured resin is used in the present invention, meaning that there is a great technical difference between the two.
When a cured resin is used, the cured resin is cured after a filler is contained therein and thus, the filler itself induces curing inhibition. Such known technologies are not good enough because the effect of the cured resin is not obtained when curing reaction is inhibited.
In addition, for example, JOP 2007-72487 describes a technology in which a filler is dispersed in a solvent without a binder resin and thereafter the solution is mixed with a solution of the binder resin to prepare a liquid application for forming the surface layer. This method is an extremely good way to improve the dispersion property of a filler and the dispersion stability and is used in the present invention. However, a single use of this method is insufficient. As described above, the protection layer in which a filler is dispersed in a cured resin is an extremely efficient way to improve the abrasion resistance of an image bearing member but there are a number of problems to be solved to obtain such effects. Therefore, the method of solving these problems has been aspired to improve the durability of an image bearing member.
As described above, the image bearing member having a protection layer in which a filler is dispersed in a cured resin has significantly improved abrasion resistance and damage resistance. Thus, the status of the surface of the image bearing member is unchanged during repetitive printing over an extended period of time. Therefore, this technology is durable for the use application in the quick printing field.
However, when a protection layer formed of a filler and a cured resin is formed on a photosensitive layer (or charge transport layer), the curing reaction of a curable resin may be inhibited due to the existence of the filler. This does not cause a problem when the dispersion property of a filler is high. However, when a filler having a bad dispersion property is used and thus agglomerates, the curing reaction is significantly inhibited and thus the filler is not sufficiently held in the cured resin. In addition, when the filler agglomerates and is not sufficiently dispersed, the number of fillers with thinly covered by the resin increases and thus the filler tends to be detached therefrom. Thereby, the abrasion resistance of the image bearing member significantly deteriorates. Furthermore, when the resin is not sufficiently cured by the curing inhibition, the damage resistance of the image bearing member deteriorates. In this case, when the filer is easy to be detached from the resin due to the decreased retaining power of the filler, the detached filler further damages the surface of the image bearing member, which accelerates deterioration of abrasion resistance and damage resistance.
In addition, agglomerated fillers have an impact on the layer quality of the protection layer. That is, a roughened surface and protruded coating deficient portions are formed. These create problems of image deficiencies in a spot manner, bad cleaning performance, etc. Furthermore, since the filler quickly settles down in a liquid application when the dispersion property of the filler is bad, the content of the filler depends on the application time and thus the uniformity of the filler contained in the protection layer decreases. As a result, locally abraded portions and electrostatic deterioration occur and images having image deficiency in a spot manner and/or uneven image density are produced. Therefore, it is extremely preferable to improve the dispersion property of a filler to form a protection layer in which the filler is dispersed in a cured resin.
Furthermore, curing inhibition is not just caused by agglomeration of the filler. When a charge transport material contained in the photosensitive layer is dissolved in a solvent contained in a liquid application of the protection layer during formation of the protection layer on the photosensitive layer, the charge transport material elutes into the protection layer and the eluted charge transport material may causes curing inhibition. A small amount of the eluted charge transport material has only a slight and limited impact with regard to the curing inhibition and improves the charge infusion property at the interface between the charge transport layer and the protection layer, which may lead to a decrease in the voltage at irradiated portions and improvement on the sensitivity in some cases. In addition, the elution of the charge transport material to the protection layer improves the attachability of the protection layer and the photosensitive layer in some cases.
However, when an extremely large amount of the charge transport material elutes into the protection layer, curing is significantly inhibited, which results in significant deterioration of abrasion resistance and damage resistance. Furthermore, when a charge transport material having a small ionization potential is contained in a photosensitive layer, no problem is created if the elution amount of the charge transport material to the protection layer is small. However, an elution amount that is excessively large tends to cause image blur and reduction in resolution in an oxidizing gas atmosphere, which may lead to significant degradation in the image quality.
In addition, when a great amount of the charge transport material elutes into the protection layer, the charge transport material itself inhibits the curing reaction and in addition absorbs the ultraviolet when the protection layer is cured by irradiation of ultraviolet, which inhibits the curing reaction inside the protection layer. Increasing the amount of irradiation of ultraviolet, and/or using a polymerization initiator, etc. more than necessary to promote the curing reaction cause a significant rise of the residual voltage and significant deterioration of the photosensitivity in most cases. This leads to side effects such as wrinkle or cracking in the protection layer or peeling-off thereof because the attachment force to the photosensitivity decreases. Thus these are not suitable solutions. Therefore, adequately limiting the elution mount of the charge transport material is preferable.
As described above, in terms of formation of a protection layer in which a filler is dispersed in a cured resin, it is extremely preferable to improve the dispersion property of the filler and adequately limit the curing inhibition caused by the filler and the charge transport material in the photosensitive layer eluted into the protection layer. However, it is extremely difficult to have a good combination of these. This is because a preferable solvent for dispersion of the filler does not necessarily match a solvent that adequately limits the elution amount of the charge transport material in the photosensitive layer
In addition, when a liquid application for the protection layer contains a large amount of a solvent in which the charge transport material is highly soluble, the elution amount of the charge transport material to the protection layer significantly increases. When a liquid application for the protection layer contains a large amount of a solvent in which the charge transport material is insoluble, the charge transport material hardly elutes into the protection layer, which may cause peeling off of the protection layer and precipitation of the charge transport material. On the other hand, the solubility of the charge transport material contained in the photosensitive layer to a solvent depends on the kind of the charge transport material. Therefore, the combination of the solvent contained in the liquid application for the protection layer and the charge transport material contained in the photosensitive layer is selected in terms that the filler is preferably dispersed and the elution amount of the charge transport material in the photosensitive layer is adequately maintained.